Digital quantification of porcine circovirus 2 (PCV-2)-infected cells in lymph nodes of pigs with natural PCV-2 systemic disease shows strong association with manual scoring and quantitative PCR

The diagnosis of porcine circovirus 2 (PCV-2) systemic disease is based on clinical signs, microscopic lesions, and semiquantitative PCV-2 assessment by in situ visualization of immunohistochemistry (IHC) or in situ hybridization labeling within lymphoid tissue lesions. We aimed to digitally quantify IHC-labeled PCV-2-infected cells in porcine lymph nodes with natural PCV-2 systemic disease (<i>n</i> = 53) and to compare these quantitative results with the subjective manual method, and also with the viral DNA copy numbers of the corresponding formalin-fixed, paraffin-embedded tissue blocks. In total, 47 of 54 (87%) cases had the same manual score by 2 independent observers. Infected cell counts increased with manual score severity, and their count could be matched to the manual method. Infected cell odds showed a significant strong positive correlation with the viral copy numbers (ρ = 0.8461, <i>P</i> < .001). Digital image analysis is efficient for PCV-2 IHC signal quantification and can make case assessment more objective and reproducible.